Implantable cardiac pacemakers have conventionally been placed in a subcutaneous pocket and coupled to one or more transvenous medical electrical leads carrying pacing and sensing electrodes positioned in a heart chamber, e.g., along an endocardial wall. A cardiac pacemaker implanted subcutaneously may be a single chamber pacemaker coupled to one medical lead for positioning electrodes in one heart chamber, atrial or ventricular, or a dual chamber pacemaker coupled to two leads for positioning electrodes in both an atrial and a ventricular chamber. Multi-chamber pacemakers are also available that may be coupled to three leads, for example, for positioning electrodes for pacing and sensing in one atrial chamber and both the right and left ventricles.
Leadless intracardiac pacemakers have recently been introduced that are wholly implantable within a ventricular chamber of a patient's heart for delivering ventricular pacing pulses. Such a pacemaker may sense R-wave signals attendant to intrinsic ventricular depolarizations and deliver ventricular pacing pulses in the absence of sensed R-waves using electrodes carried along the housing of the pacemaker. While single chamber ventricular pacing may adequately address some patient conditions, other conditions may require atrial or dual chamber atrial and ventricular pacing in order to maintain a regular heart rhythm.